Rolling element bearing having starved lubrication conditions

ABSTRACT

A rolling element bearing comprises an inner ring and an outer ring which are each provided with a raceway, and a series of rolling elements which are in contact with the raceways of each ring. A lubricant film is provided in the contacts between the rolling elements and the raceways, which film forms a lubricant meniscus at the inlet side of each contact. In the bearing starved lubrication conditions prevail. The surface of each rolling element has minute recesses filled with a lubricant quantity, said recesses being flattened in the contact area defined by the contact between the rolling elements and the rings and thereby releasing lubricant at the inlet side of each contact resulting in a displacement of the meniscus further away from said contact. This meniscus displacement results in an increased lubricant film thickness in each contact area, thus improving the lubricating conditions in the bearing.

The invention is related to a rolling element bearing which operatesunder starved lubrication conditions. In such cases, not enoughlubricant is available in the inlet side of the contact for entrainmentsuch that a lubricant film of sufficient thickness in the contact areasdefined by the contacts between the rolling elements and the rings isformed. In normal operating conditions the film thickness between ringsand rolling elements is determined by the geometry of the contacts, thelubricant properties, such as viscosity, and the entraining velocity. Instarved lubrication conditions the quantity of lubricant also plays arole.

In that case the lubricant film thickness in the contact areas maybecome so small that the surfaces, or surface asperities, of the rollingelements and the rings will touch. As a consequence, the life of thebearing is reduced by adhesive wear or surface distress.

The reduction of film thickness can also lead to a reduction of lifecaused by contamination. Larger particles can no longer freely pass thecontact but will cause dents which may reduce bearing life again.

Starved lubrication conditions may result from several reasons. Forinstance, the amount of lubricant in the bearing may simply be toosmall. Starved lubrication conditions may also occur due to a lack ofproper re-flow. A roller overrolling a running track in a bearing causesthe lubricant to partially flow out of the running track. If the speedof the following roller is large, or the viscosity of the lubricant ishigh, there is no time for the lubricant to flow back into the runningtrack. The remaining amount of lubricant for this roller is then toosmall, which means that this roller cannot form a film of the requiredthickness.

This problem is even aggravated in grease lubricated bearings, in whichthe grease is pushed to the sides of the running track and only a smallamount remains or flows inside the running track.

The object of the invention is to provide a solution to this problem, insuch a way that, even under starved lubrication conditions, the bearingstill may have an acceptable life. This object is achieved by means of arolling element bearing, comprising an inner ring and an outer ringwhich are each provided with a raceway, and a series of rolling elementswhich are in contact with the raceways of each ring, a lubricant filmbeing provided in the contacts between the rolling elements and theraceways, said lubricant film at the inlet side of each contact forminga lubricant meniscus, said lubricant meniscus being defined as thetransition from the non-lubricated phase to the lubricant phase, whereinstarved lubrication conditions prevail, the surface of said each rollingelement having minute recesses filled with a lubricant quantity, saidrecesses being flattened in the contact area defined by the contactbetween the rolling elements and the ring and thereby releasinglubricant at the inlet side of each contact resulting in a displacementof the meniscus further away from said contact, said meniscusdisplacement resulting in an increased lubricant film thickness, in eachcontact area.

Due to the increased film thickness in the contact areas, the risk ofmetal-to-metal contacts decrease and the life of the bearing increases.

It is observed that rolling element bearings have balls with minuterecesses are known per se. However, it was not recognised that suchrolling element bearings do behave better under starved lubricationconditions. According to the invention therefore, a rolling elementbearing is provided having minute surface recesses in combination withthe starved lubrication conditions, that is a lubrication film thicknesswhich usually, in the case of non-recessed surfaces, would be too smallto separate the contact surfaces fully.

As indicated, in the case of starved lubrication conditions too littlelubricant may be available in the contacts between the rings and therolling elements for obtaining an adequate separation of the surfacesthereof. Such starved lubrication conditions can be defined by using thepressure gradient at the position of the meniscus. In case sufficientlubricant is available, a fully flooded contact is obtained, which ischaracterized by a zero pressure gradient at the starting point of thepressure build-up. In the case of starved lubrication conditionshowever, this pressure gradient is non-zero. Insufficient lubricant isavailable to obtain a meniscus located at the required distance from thecontact. The lubricant content defined between the meniscus and theadjoining surfaces of the rolling elements and the ring is notsufficient to obtain adequate separation thereof at the location of thecontact.

Another way to define starvation is considering the filled fractioncontent which is defined as the ratio of lubricant film thickness andthe gap. The meniscus is then defined as the point where the filmthickness deviates from the gap. When this point is close to theHertzian contact then the contact is referred to as starved.

According to the invention however, this oil content is supplied byminute additional amounts of oil emanating from the surface recesseswhich are flattened somewhat under load at the contact. These minuteamounts then move the meniscus away from the contact as addressedbefore, which results in a better separation of the contact surfacesunder starved conditions.

The recesses can have any suitable shape, and can be arranged accordingto any suitable pattern.

As explained before, the improved lubrication effect under starvedlubrication conditions is obtained in case the recesses contain anamount of lubricant. In order to promote the availability of lubricantwithin the recesses, preferably the surface energy of the recesses isdifferent from the surface energy of the regions surrounding the pits.

As an example, the surface of the rolling elements is coated and therecesses are uncoated.

In particular, the coating is a diamond like carbon coating, and thepits comprise steel material.

The invention will now be described further with reference to the graphsshown in FIGS. 1 up to 5.

FIG. 1 shows schematically the film thickness and pressure in a singlecontact under sufficient lubricant conditions, in a cross-sectionperpendicular to the axis of a bearing.

FIG. 2 shows this film thickness and pressure under starved lubricationconditions.

FIG. 3 shows an aggregation of FIGS. 1 and 2.

FIG. 4 shows the film thickness for sufficient lubricant conditions, ina cross-section perpendicular to the raceway.

FIG. 5 shows the film thickness for starved lubrication conditions.

In the figures, H denotes the gap between the contacting surfaces. Incase of fully flooded conditions this corresponds to the film thickness.In case of starved condition this corresponds to the film thickness inthe pressurised region.

In the normalised graph of FIG. 1, the dashed line is related to thelubricant film thickness in the contact between a rolling element andthe raceway for a non-recessed rolling element. At the contact, the realthickness of the lubricant layer amounts to about 0.14 micrometer.

The fully drawn line represents the film thickness in the case of arolling element having a recess travelling through the contact where asnap shot is taken at the pit position X=0. As will be clear, thethickness of the lubricant layer inside of the contact is hardlyinfluenced by the presence of the pit.

The results of FIG. 2 show that in the case of starved lubricationconditions the thickness of the lubricant layer at the contact isdrastically decreased. For a non-recessed rolling element, the thicknessis about 0.036 micrometer, which usually is too small to fully separatethe surfaces of the rolling element and the raceway from each other.Thus, the surface asperities touch each other, leading to a reducedbearing life.

In the case of a recessed rolling element bearing, the thickness of thelubricant layer of the contact is however considerably larger, despitethe generally starved lubrication conditions of the bearing as a whole.The fully drawn line, related to a rolling element bearing with a pit atthe position X=0, at the inlet side of the contact amounts to about 0.06micrometer. This favourable effect is caused by the delivery of oil fromthe pit into the oil content which is present in front of the rollingelement or, in other words, upstream thereof. This oil content isdelimited by the surfaces of the rolling element, the raceway and ameniscus.

In FIG. 3, a direct comparison is made between the differences between arecessed and non-recessed rolling element at the location of thecontact, for fully flooded and starved lubrication conditions.

FIGS. 4 and 5 show the film thickness in a cross section perpendicularto the raceway for fully flooded conditions and for starved lubricationconditions.

As will be clear from FIGS. 1 and 4, the presence of a recess at theposition Y=0 in the fully flooded case does not make much difference,except at the position of the recess itself.

As is clear from FIG. 2, the upstream film thickness in the starved caseis better in the case of a recessed rolling element surface compared toa smooth rolling element surface.

1. Rolling element bearing, comprising an inner ring and an outer ringwhich are each provided with a raceway, and a series of rolling elementswhich are in contact with the raceways of each ring, a lubricant filmbeing provided in the contacts between the rolling elements and theraceways, said lubricant film at the inlet side of each contact forminga lubricant meniscus, wherein starved lubrication conditions prevail,the surface of each rolling element having minute recesses filled with alubricant quantity, said recesses being flattened in the contact areadefined by the contact between the rolling elements and the rings andthereby releasing lubricant at the inlet side of each contact resultingin a displacement of the meniscus further away from said contact, saidmeniscus displacement resulting in an increased lubricant film thicknessin each contact area, wherein the lubricant meniscus forms a non-zeropressure gradient with respect to the surface of the raceway.
 2. Bearingaccording to claim 1, wherein the surface energy of the recesses isdifferent from the surface energy of the regions surrounding therecesses.
 3. Bearing according to claim 2, wherein the surface of therolling elements is coated and the recesses are uncoated, or reverse. 4.Bearing according to claim 3, wherein the coating is a diamond likecarbon coating, and the recesses comprise steel material.
 5. Bearingaccording to claim 1, wherein the lubricant film thickness obtainedafter displacing the meniscus further away from the contact amounts toat leas 0.01 μm the middle of the contact.
 6. Bearing according to claim1, wherein the recesses form an isotropic surface pattern.
 7. Bearingaccording to claim 1, wherein the recesses form an anisotropic surfacepattern.
 8. Bearing according to claim 1, wherein each rolling elementor ring carries a layer of lubricant as a result of surface tension. 9.Bearing according to claim 1, wherein the recesses have an arbitraryshape.
 10. Bearing according to claim 9, wherein the recesses comprisetransverse grooves.